This article is titled “Caliber Confusion” because calibers are confusing. Don’t allow anyone to make you feel stupid if you don’t intuitively understand this stuff — because there’s nothing intuitive about it.

Safety Note

It is very dangerous to fire ammunition of the wrong size for your gun. Most firearms have the correct caliber stamped on the side of the barrel or frame. If yours does not, don’t simply guess at the correct ammunition size. Take the gun to a gunsmith or to the gun store from which you purchased it, and have an expert measure your firearm’s chamber.

Generally speaking, caliber refers to the size of the bullet a gun will fire, and also usually refers specifically to the bullet’s diameter or to the bore size of the gun that fires it. But due to all sorts of vagaries in the ammunition-manufacturing world, the nominal caliber designation isn’t always the actual bullet diameter or chamber measurement. Nor is the bullet diameter the only factor that matters, as we will see in the discussion below.

Sometimes caliber is measured on the imperial scale, by decimal fractions of inches. Other times it is measured on the metric scale, usually in millimeters. In either case, the first number you see isn’t always all the information you need to discover in order to know if the ammunition will work in your gun.

Sometimes, instead of one simple number, the ammunition size and type will be represented by two groups of numbers. In such cases, the first number usually indicates the bullet diameter, while the second number often (but not always) represents the length of the cartridge, measured from base to case rim.

Caliber numbers are usually followed by words or letters to create the complete name of the cartridge. These letters often represent a brand name or an abbreviation for the name of the company that first introduced the round, or give more information about the length of the cartridge. These letters are as important as the caliber number. In most cases, it isn’t enough to know the initial numbers. You’ll need to know the letters which follow them as well.

Below follows a whirlwind tour of some common calibers and cartridges. It is not a comprehensive list, merely a quick overview of some caliber-related facts which might interest a beginning or intermediate shooter.

Handgun cartridges are relatively straightforward, compared to the unbelievable things that the shotgun and rifle ammunition manufacturers have done to us. But there is still plenty of opportunity to get mixed up.

One important thing to remember about handgun ammunition is that ammunition designed to be fired in semi-automatics is generally different in shape from ammunition designed for revolvers. What this means is that there are some rounds which, on paper, seem as though they should be identical — but they look different when seen up close, and are designed to fit in different types of guns.

The main difference between handgun and rifle ammunition is simply that rifle rounds are usually (but not always) more powerful than pistol rounds. They usually have longer cases, and the cases often hold more gunpowder.

Starting with the lowly .22, then, let us begin. The three .22 rounds discussed below are all rimfires.

The most common .22-caliber round is the .22 Long Rifle, which is often abbreviated to .22 LR. Don’t let the word “rifle” in the name confuse you, because this ammunition is fired from both handguns and rifles. When someone refers to shooting a .22, they will usually be talking about the .22 LR. This ammunition is plentiful, easy to find, and very inexpensive. It has very little recoil and isn’t as loud as many other cartridges so it is very comfortable to shoot. All of this makes it an ideal round for beginners who wish to learn to shoot well.

Rimfire

Ammunition in which the primer is located in the bottom rim of the case. Typically, rimfire rounds are smaller calibers than centerfire rounds. Rimfire is often used in casual conversation to refer exclusively to the .22 Long Rifle cartridge or to guns that fire it.

Next up — but actually a step down in power — is the .22 Short. This one is the same circumference as the .22 LR, but comes in an even shorter case. There’s not a lot of power behind the .22 Short, but it’s a fun one to shoot. When first writing this page, I commented that I’d never seen a rifle which uses this round. An alert reader, Tom Holiday, sent me the following:

“When I was a kid, (50’s 60’s) the shooting gallery at the state and county fairs would use guns chambering .22 shorts. These were pump guns. My mother has a .22 crack shot single shot carbine that shoots all .22 rounds. Mom’s gun is at least 80 years old. I would imagine that any single shot breech loader would be able to fire the .22 short. They may not be able to eject them, but the spent cases can easly be removed by hand or with a knife blade.”

The .22 Long Rifle and .22 Short are both the same diameter.

The .22 Winchester Magnum Rimfire, or .22 WMR, is often called the .22 Magnum or .22 Mag in casual speech. The .22 Magnum is slightly larger in diameter than the .22 LR or .22 Short. The case is also somewhat longer and holds more powder, giving the round a little more punch than the .22 LR. It is used in both rifles and handguns. A revolver chambered for .22 Magnum will physically accept a .22 LR round, but these are different calibers, and it is dangerous to fire a .22LR in a gun designed to fire .22 Magnum. 1 Also pointless, because it won’t be very accurate. Like .22 LR, .22 WMR can be fired from both handguns and rifles.

Centerfire

Ammunition in which the primer is located in a small cup in the bottom center of the case.

Larger in diameter than the .22 calibers, but less powerful, is the .25 Auto Colt Pistol, usually called .25 ACP or .25 Auto. The .25 ACP is a centerfire round unique to handguns. If you’re looking for a defense round, stay away from this one. Although quite common in cheap handguns, it really isn’t powerful enough to do what you need it to do.

Of the .25 Auto, firearms great Jeff Cooper once famously quipped, “… carry a 25 if it makes youfeel good, but do not ever load it. If you load it you may shoot it. If youshoot it you may hit somebody, and if you hit somebody – and he finds out aboutit – he may be very angry with you.”

The .32 ACP or .32 Auto was among the many cartridges developed and popularized by John Moses Browning and the Colt company back around the turn of the last century. In Europe, this round is called the 7.65mm Browning. Today it is used in small, lightweight pistols, especially those small enough for pocket carry, but most firearms instructors will tell you that .32 ACP is not powerful enough to be used for dedicated self-defense.

Designed for revolvers, the .32 H&R Magnum round was introduced in 1983. It isn’t the same thing as the .32 ACP. A handful of firearms manufacturers produce revolvers chambered for the .32 H&R Magnum.

The Really Confusing Handgun Calibers

With the .380 ACP or .380 Auto, we enter the realm of truly confusing caliber facts. Remember that the cartridge numbers often, but not always, indicate the diameter of the bullet? The .380 Auto is one of the exceptions. Its bullet actually measures .355″ (9mm) in diameter. Because its bullet diameter is 9mm, but the overall length of the cartridge is shorter than other 9mm ammunition, .380 ACP is sometimes called the 9mm Short. Other names for it are 9mm Kurz (kurz means short in German), or 9mm Corto (Corto means short in Italian). It’s also sometimes called the 9mm Browning because its inventor was John Moses Browning. The .380 ACP round is widely believed to be the minimal round acceptable for self-defense, and it was the favored police round throughout Europe for most of the 20th century.

Good Grief! How Many 9mm Cartridges Are There?

Good question! Generally speaking, if you hear someone refer to a 9mm, they will mean the 9mm Luger round.

Another round in this size class is the 9x18mm Makarov, often simply called the 9×18. This one was pretty rare in America until a few years following the breakup of the USSR but has become more familiar to American buyers with the popularity of Makarov pistols. Lengthwise, the 9×18 falls about halfway between the .380 ACP and the more common 9mm Luger, but in terms of performance, it’s most similar to the .380 ACP. A confusing factoid: although the 9×18 is called a 9mm round, it is really not 9mm. Its bullet is slightly larger than 9mm, measuring .364″ rather than the 9mm-equivalent .355″

Despite the incredible number of cartridges which fire bullets 9mm in diameter, most of the time, when someone refers simply to a 9mm, they are talking about the very common 9mm Luger round, so named in honor of its inventor, Georg Luger. Other names for this same round are 9mm Parabellum, or 9mm Para, or 9x19mm, or 9mm NATO. The 9mm Luger is perhaps the least expensive of all self-defense rounds. It is more powerful than the anemic .380 ACP, but like the .380 ACP it easily lends itself to firearm designs which are small and light enough to carry comfortably. It is widely used in police work throughout the world, and is one of the most popular calibers for concealed carry.

Is .38 Super the same as .380 Auto?

No. These are entirely different rounds. Although both are intended to fire in semi-automatic handguns, .380 Auto has an overall length of slightly under an inch, while a .38 Super cartridge usually measures over an inch long. Even more confusingly, although they begin with the same numbers, they aren’t the same diameter, and neither actually uses .38″-diameter bullets.

To avoid further confusion, I’m not even going to talk about the 9mm Largo cartridge, a Spanish round rarely seen in America and sometimes referred to as the 9×21. Just be aware that it isn’t the same thing as the 9mm Luger.

We still haven’t yet left the realm of the 9mm calibers. Next up is the .357 Sig, a young cartridge which was first introduced in 1994 and designed by SIGARMS (the folks who produce Sig Sauer pistols) in partnership with Federal Ammunition company. Created to fire in semi-automatic handguns, it is intended to mimic the ballistic performance of .357 Magnum revolver ammunition. Despite its name, the .357 Sig does not use .357″ diameter bullets, but rather uses 9mm / .355″ diameter ones. It still remains to be seen whether .357 Sig is a permanent fixture on the defensive handgun market, or just a flash in the pan — but it does have an enthusiastic following.

.38 Super Auto, commonly called simply .38 Super, was introduced in 1929 and was popular for many years. However, it was steadily diminishing in popularity and perhaps on the edge of extinction when a new shooting sport called IPSC revived it and gave it a place in history. Now it is once again among the most popular cartridges. The .38 Super is widely used in competition firearms to provide power nearly equivalent to a .45 ACP, but with the lighter recoil and magazine capacity more typical of the 9mm. Despite the name, .38 Super does not fire bullets .38″ in diameter. The bullets it uses are .355″ or .356″ across.

Safety Note

Revolvers designed to fire .357 Magnum can also shoot .38 Special ammunition. But it doesn’t work the other way around. Even though .357 Magnum ammunition will fit into some older .38 Special firearms, take note: 2

Revolvers designed to fire .38 Special should never be used to fire .357 Magnum ammunition.

This is because .357 Magnum ammunition is much more powerful than .38 Special, and therefore requires a sturdier gun for safety’s sake.

While the .38 Super is a semi-auto round, .38 Special is designed for revolvers. It is probably the most popular revolver cartridge ever produced, and is easily found in most parts of the world. Despite the name, this caliber actually uses .357″ bullets. While any short-barreled revolver may be called a “snubby” by aficionados, most folks immediately think of a .38 Special caliber revolver when they hear the word snubby.

As with many other rounds, the .38 Special comes in standard and +p variants. The .38 Special has a long history, which means there are a lot of revolvers out there which were produced to fire this round in the days before modern metallurgy. These older revolvers are not designed to handle the more powerful ammunition called +p (think, plus power) which became available in the early 1970’s. As a result, older .38 Special revolvers should fire only the standard rounds, and steer clear of ammunition marked +p. Even some modern revolvers, most notably the super-lightweight alloys, are not designed to handle large volumes of +p ammunition. If in doubt, read the owner’s manual or call the manufacturer before using ammunition marked +p.

The nice thing about the .357 Magnum round is that revolvers designed to fire it can also fire .38 Special ammunition. What this means is that gun owners who purchase a .357 Magnum revolver will be able to use the less-expensive .38 Special as a practice round, while reserving the more powerful and more expensive .357 Magnum rounds for self-defense. Be aware, however, that repeatedly firing .38 Special ammunition through your .357 Magnum frame requires a special emphasis on regular, deep cleaning. Otherwise, a ring of lead residue will build up within the chambers. If enough rounds are fired between effective cleanings, the residue may become so stubborn as to be considered permanent, and its presence will prevent the ability to chamber the longer .357 Magnum round in that revolver.

.40 S&W and 10mm Auto have an interesting relationship. 3 The idea of a .40″ / 10mm diameter semi-automatic cartridge had been around for many years, but it wasn’t until 1983 that a commercial version of the 10mm appeared — and it was another five years after that before handguns designed to fire it became widely available.

During this time frame, many law enforcement agencies issued 9mm Luger weapons for their departments. Following a disastrous shootout in Miami, the FBI determined that they needed to use a more powerful round, and elected to go with the 10mm Auto. However, the FBI administration soon came to believe that the 10mm was too much gun for many of its agents. For this reason, the FBI began downloading their 10mm Auto ammunition, putting less powder (and thus less power) in the long cases.

In part because of the FBI’s search for a less-powerful 10mm (or at least a reliable semi-auto in that caliber class), Smith & Wesson introduced the .40 S&W cartridge in the early 1990’s, using a shorter case but a bullet the same diameter as the 10mm Auto. This allowed pistols designed around the .40 S&W cartridge to be somewhat smaller and more concealable than those designed around the 10mm Auto. As soon as the .40 S&W was introduced, the FBI adopted it and many other law enforcement agencies swiftly followed suit. Today the .40 S&W may be the most commonly used police round in America.

First came the .44 Russian, a very old cartridge developed back in the 1870’s by Smith & Wesson for the Russian Army under the czars. Initially developed as a black-powder round, it successfully made the transition to smokeless powder and today is very popular for Cowboy-Action Shooting.

Note

Revolvers designed around the .44 Special can also fire .44 Russian. Those designed around the .44 Magnum can also fire both .44 Special and .44 Russian.

Introduced around the turn of the last century, the .44 S&W Special (commonly called simply .44 Special) uses a somewhat longer case than the older Russian round. When carried in a compact revolver, .44 Special can be a good choice for concealed carry. Like the .44 Russian, .44 Special is also very popular in the Cowboy Action sports.

“This is a .44 Magnum, the most powerful handgun in the world … Do you feel lucky, punk?” Despite the claim in Clint Eastwood’s famous lines, the .44 Magnum (its full name is .44 Remington Magnum) was not and is not the absolute most powerful handgun cartridge. 4 It is, however, a very powerful round and the popular mystique surrounding it makes it even more so. Though it is an excellent hunting round, .44 Magnum is really too powerful to use for self-defense: it is difficult to shoot rapidly, and there’s a high possibility of the bullet going straight through the intended target to hit innocent passersby.

No matter what the old guys in the gun shop tell you, .45 Auto or .45 ACP will not send an assailant flying across the room if you hit him in the pinkie finger. It is, however, a very good defense round which makes satisfyingly large holes in the target.

ACP?

ACP means “Automatic Colt Pistol.” You’ll find it used to designate many different cartridges which were originally designed by the Colt Firearms Company to be fired through semi-automatic or automatic firearms.

The .45 ACP was among the many cartridges first developed by John Moses Browning for the Colt Company around the turn of the 20th century, and has become one of the most popular and successful rounds ever invented. In part this is because of the tremendous continuing popularity of 1911-pattern pistols designed to fire .45 ACP. Even though there are 1911 variants which fire other calibers and even though there are many other handguns designed around the .45 ACP cartridge, the .45 ACP and the 1911 pistol are closely linked in the minds of most shooters.

Introduced just a few years ago, the .45 GAP or .45 Glock was designed to achieve similar ballistic performance as the .45 ACP, but be fired from smaller, more concealable pistols. To achieve this, it uses a somewhat shorter case than the .45 ACP. Guns sized for .45 GAP are thus generally more suitable for shooters with small hands than guns sized for .45 ACP ammunition. A few other companies have picked up the cartridge and designed firearms around it, so chances are that it will remain on the market for at least a while.

An old round designed for revolvers, the .45 Colt (sometimes incorrectly called the .45 Long Colt) is still popular today. In fact, it is the oldest centerfire handgun cartridge still in regular use. It’s most commonly found at Cowboy-Action games, but can also be found in several more modern revolvers. It is not the same as the .45 ACP: the case is longer and has a higher volume, making it potentially a more powerful round.

The .50″ calibers are all outside the realm of reasonable defense weapons, mostly because of controllability and follow up speed. These really aren’t for beginners in any case.

The only thing it seems necessary to point out here is that the .50 AE (or .50 Action Express) designed for handguns is radically different from, and weaker in power than, the .50 BMG round designed for rifles. The recently-introduced .500 Magnum S&W is a revolver round in the same caliber class, but packs considerably more power than the shorter .50-caliber semi-auto rounds.

Note: If you are looking for a little more information about handgun ammunition sizes and types than this article provided, Genitron’s ammunition page is definitely the next place you need to look. The page contains excellent pictures of various rounds, on a measured background grid for easy size comparison.

Forget everything you just learned about caliber. Shotguns are different beasts entirely.

The gauge is the single most important number to remember, but a box of shotgun ammunition provides a lot more information that you will need to understand. Because shotguns are very versatile firearms, ammunition manufactured for them comes in a very wide variety of types. Within the types, there’s also a large variation in size and power.

Gauge

Shotgun ammunition is measured in gauge rather than in caliber. Gauge refers to how many lead balls the same diameter as the bore would equal one pound. In the case of a 12-gauge shotgun, a lead ball that exactly fits within the barrel weighs 1/12th a pound. So it would take twelve lead balls the size of the shotgun’s bore to equal a pound.

Remember

The smaller the gauge number, the bigger the barrel’s internal diameter will be.

Counterintuitively, the smaller the bore, the more lead balls the same size it would take to equal one pound. So a higher gauge number means the internal diameter of the barrel is smaller, while a smaller gauge number means the internal diameter of the barrel is bigger.

Thus, a 12-gauge bore is bigger than a 20-gauge bore.

In the past, shotguns were made in many different gauge sizes. Today, the most common shotgun sizes are 12-gauge and 20-gauge. There are also 10-gauge, 16-gauge, and 28-gauge shotguns, as well as .410 bore shotguns. 5

It can be difficult to find ammunition for the less common gauges.

The gauge number is the most important number on the shotgun ammunition box. If you have a 12-gauge shotgun, only 12-gauge ammunition will work in it. A 20- or 28-gauge shell accidentally loaded into a 12-gauge shotgun can become entirely lodged in the barrel. The next shot fired could result in a potentially lethal mess.

Safety Note

It is very important that you use the shell length your shotgun was designed for, or shorter, but nothing longer. If you use a longer shell, it may create too much pressure for your shotgun to handle.

Shell Length

The second most important number you will see on the box is the overall length of the shotgun shell. Not all lengths will feed in all shotguns. The common lengths are 2-3/4 inches, 3 inches, and 3-1/2 inches.

The longer the shell, the more shot pellets and powder it can contain. For this reason, shotguns which are designed to load a shorter shell should never be used to fire a larger shell, even if the larger shell physically fits within the gun. The gun may not be able to handle the higher pressures a more powerful ammunition produces.

Shell Type

There are three basic types of shells:

High brass shells are shells that have a brass base which extends up the shell body by about three-quarters of an inch.

Low brass shells are characterized by a relatively narrow band of metal around the base of the shell. Low brass ammunition is generally less powerful than high brass.

Activ shells are formed entirely of plastic, except for a miniature metal button which holds the primer in the center of the case head. Activ shells are useful for hunters and others whose ammunition might get wet in the field, because they are nearly impervious to rust.

Dram Equivalent (power)

To figure out how powerful the ammunition is, look at the number marked “dram equivalent.” Originally, drams were a black powder weight measure. Although shotgun ammunition uses modern smokeless powder, in order to standardize measurements, shotgun ammunition manufacturers use dram equivalents to indicate how much power the load has. The quantity of smokeless powder in the load is compared to the amount of black powder that would produce the same velocity with the same projectile(s).

The higher the dram equivalent number, the more energy the ammunition has and the faster the shot will travel downrange.

Shot Size

Okay, here’s where it gets a bit more confusing. Shot sizes range from the smallest, No. 9 which is .08 inches in diameter, up to 000 buckshot at .36 inches in diameter.

Shot

Multiple pellets contained in the shell and sent downrange when the shotgun is fired. No matter how many pellets there are, shot is pluralized without adding an ‘s’ to the end: “a handful of shot.” “The case was filled with #6 shot.”

What this means is that No. 4 Shot is substantially different in size from No. 4 Buckshot. Memory cue: remember that Buckshot, the bigger word, is usually bigger than Shot, the smaller word.

Shot Type

Shotgun pellets are formed from heavy, dense metals. The name of the metal will usually be marked on the box. If the shot is formed from lead, the pellets will often be coated with copper or nickel plate to preserve their round shape during flight.

Be aware that because of environmental concerns about lead, EPA regulations prohibit hunting waterfowl with lead shot. Unless you are hunting larger game, before you go afield with your shotgun, you’ll need to find ammunition loaded with non-lead alternatives, such as steel, tungsten iron, or bismuth shot.

Amount of Shot

The number of projectiles inside an individual shotgun shell is indicated by their weight in ounces for all but the large steel and buckshot sizes. In the larger sizes, instead of a weight in ounces, you’ll find the number of shot or buckshot which will fit within one shell.

Slug

Not all shotgun ammunition contains shot. Sometimes, instead of shot, the shell will contain a slug. A slug is a single, very large bullet. Sometimes the slug will have rifling on the outside, sometimes not. The slug might be partially encased in a sabot (pronounced sah-bo), which is a (usually plastic) covering designed to improve the way the slug fits snugly within the bore.

Exotic Ammunition Types

Shotgun shells can be loaded with nearly anything, ranging from less-lethal types to types that are most decisively lethal.

Less-lethal types include rubber buckshot, bean bags, and pepper balls. These are each somewhat less likely to kill someone than traditional ammunition, but despite common perception, it is entirely possible to kill someone any of these. Because it is possible to kill with less-lethal ammunition, shooting someone with this type of ammunition will usually be treated the same, legally, as shooting them with traditional ammunition. Less-lethal ammunition is most often used by law enforcement in situations where extraordinary action is required in order to contain a riot or subdue a suspect. Department regulations generally require that if less-lethal ammunition is used, officers have immediately available backup who are ready to fire with traditional weapons. This is because less-lethal ammunition does not always stop determined criminals. Because less-lethal ammunition does not reliably stop determined attackers, or those who are hyped up on drugs, less-lethal ammunition is not recommended for self-defense use.

Some shotgun cartridges are loaded with flechettes instead of shot. Flechettes are small, steel, pointed dart-like projectiles with aft stabilization fins. Despite online mystique, these aren’t best for defense because each flechette has a very low cross sectional area, and because only a few flechettes can be loaded into each shotshell. That makes flechettes an inferior choice for home defense when compared to buckshot.

There are other various exotic shotshells: incendiary Dragon’s Breath, bird bombs, ceramic slugs, bolo projectiles, and so on.

Many of the most exotic shotshells are handloaded by amateurs, rather than coming from factory production. It can be very dangerous to fire ammunition that someone else has constructed. If you want to sample exotic shotgun loads, try to stay with factory-produced ammunition.

Other Information

There is often other information on the ammunition box. Most helpfully, many manufacturers will mark their ammunition with a suggested use. If all the numbers above confuse you greatly, go with the manufacturer’s suggested use. These markings won’t lead you too far astray.

When looking for your first practice ammunition, find something marked “light.” Light loads generally have less power in them and will kick less. 7

Some single-action revolvers (and a few rare double-action types) allow the user to convert a .22 Magnum revolver to fire .22 LR ammunition as well. Such conversions are perfectly functional and accurate; the caution above applies to firearms that are designed solely to fire .22 Magnum ammunition. ↩

Modern .38 Special firearms will not chamber .357 Magnum ammunition, and the .357 Magnum round was specifically designed to prevent that possibility. However, there are older, inexpensive .38 Special revolvers still available on the used market which have chambers bored all the way through, making it possible to physically insert a .357 Magnum round into these guns. These older firearms were not designed to cope with the stronger pressure of the modern .357 Magnum round. ↩

By the way, heavy isn’t the opposite of light when seen on a shotgun ammunition box. Heavy either means there is more shot per shell than one would expect, or it means the shot is made of an alloy that weighs more than plain steel. ↩

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